= Atom-cloud gravity-gradient measurement
A laboratory <gravity gradiometry> experiment used two vertically separated clouds of <chemical atoms> in <vacuum>, launched into <free fall>. Shared <laser> beams applied three pulses ($\pi/2,\pi,\pi/2$), splitting and recombining <matter waves>. Balanced <photodiodes> measured probe-light absorption to infer internal-state populations; scanning the last pulse's <wave phase> produced an <interference> fringe.<Sensitive absolute-gravity gradiometry using atom interferometry by J. M. McGuirk and others>[§I, pp. 033608-1–2; §II E 1, p. 033608-6; §II F 1, p. 033608-7]{ref}
The difference of fringe <wave phases> obeys $\delta\phi=\mathbf k_{\mathrm{eff}}\cdot\Delta\mathbf g\,\tau^2$, with effective <wave vector> $\mathbf k_{\mathrm{eff}}$ and pulse spacing $\tau$: shared reference motion cancels, leaving differential <gravitational acceleration>. Eight lead bricks, each approximately $12.5\,\mathrm{kg}$, were placed symmetrically around the lower chamber, about $0.2\,\mathrm m$ from the fountain apex. The measured signal was $(8.1\pm2.1)\times10^{-9}g$, versus a calculated $8.2\times10^{-9}g$, where $g$ denotes terrestrial <gravitational acceleration>.<Sensitive absolute-gravity gradiometry using atom interferometry by J. M. McGuirk and others>[§I, p. 033608-2; §III A 2, p. 033608-10, mass detection]{ref}
Controls deliberately shook the shared reference reflector and compared actively isolated and unisolated operation. Tilting tests exposed sensitivity to <Coriolis acceleration> and <centrifugal acceleration>. Thus shared translation is rejectable, while rotation and finite measurement uncertainty remain limitations. The brick signal detects spatially varying <gravity>; it does not establish a precision value of the <gravitational constant>.<Sensitive absolute-gravity gradiometry using atom interferometry by J. M. McGuirk and others>[§III A, p. 033608-9; §III B, pp. 033608-10–11]{ref}
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